There are two commonly used techniques to fracture in a completion method. FIG. 1 shows a borehole 10 that has a casing string 12 that is cemented 14 in the surrounding annulus 16. This is normally done through a cementing shoe (not shown) at the lower end of the casing string 12. In many cases if further drilling is contemplated, the shoe is milled out and further drilling progresses. After the string 12 is cemented and the cement 14 sets a perforating gun (not shown) is run in and fired to make perforations 18 that are then fractured with fluid delivered from the surface followed by installation and setting of packer or bridge plug 20 to isolate perforations 18. After that the process is repeated where the gun perforates followed by fracturing and followed by setting yet another packer or bridge plug above the recently made and fractured perforations. In sequence, perforation and packer/bridge plug pairs 22, 24; 26, 28; 30, 32; and 34 are put in place in the well 10 working from the bottom 36 toward the well surface 38.
A variation of this scheme is to eliminate the perforation by putting into the casing wall telescoping members that can be selectively extended through the cement before the cement sets to create passages into the formation and to bridge the cemented annulus. The use of extendable members to replace the perforation process is illustrated in U.S. Pat. No. 4,475,729. Once the members are extended, the annulus is cemented and the filtered passages are opened through the extending members so that in this particular case the well can be used in injection service. While the perforating is eliminated with the extendable members the cost of a cementing job plus rig time can be very high and in some locations the logistical complications of the well site can add to the cost.
More recently, external packers that swell in well fluids or that otherwise can be set such as 40, 42, 44, 46, and 48 in FIG. 2 can be set on the exterior of the string 49 to isolate zones 50, 52, 54, and 56 where there is a valve, typically a sliding sleeve 58, 60, 62 and 64 in the respective zones. The string 49 is capped at its lower end 67. Using a variety of known devices for shifting the sleeves, they can be opened in any desired order so that the annular spaces 68, 70, 72 and 74 can be isolated between two packers so that pressurized frac fluid can be delivered into the annular space and still direct pressure into the surrounding formation. This method of fracturing involves proper packer placement when making up the string and delays to allow the packers to swell to isolate the zones. There are also potential uncertainties as to whether all the packers have attained a seal so that the developed pressure in the string is reliably going to the intended zone with the pressure delivered into the string 49 at the surface. Some examples of swelling packer are U.S. Pat. Nos. 7,441,596; 7,392,841 and 7,387,158.
In some instances the telescoping members have been combined with surrounding sleeves of a swelling material to better seal the extended ends of the telescoping members to the formation while still leaving open the remainder of the annular space to the formation in a given zone. Some examples of this design are U.S. Pat. Nos. 7,387,165 and 7,422,058. US Publication 2008/0121390 shows a spiral projection that can swell and/or be expanded into wellbore contact and leave passageways in between the projections for delivery of cement.
U.S. application Ser. No. 12/463,944 filed May 11, 2009 and entitled Fracturing with Telescoping Members and Sealing the Annular Space shows a technique to pinpoint the applied frac pressure to the desired formation while dispensing with expensive procedures such as cementing and annulus packers where the formation characteristics are such as that the hole will retain its integrity. The pressure in the string is delivered through extendable conduits that go into the formation. Given banks of conduits are coupled with an isolation device so that only the bank or banks in interest that are to be fractured at any given time are selectively open. The delivered pressure through the extended conduits goes right to the formation and bypasses the annular space in between. Beyond that the string exterior can have a covering of a swelling material such as rubber or a shape memory polymer, either of which can fill the annular gap and replace the traditional and expensive cement job.
Also relevant are: US Publication 2006/0201675; U.S. Pat. Nos. 7,059,407; 6,957,701; 6,672,405; 6,575,247; 6,543,538; 6,520,255; 6,394,184; 5,765,642; L. East, Packerless Multi-stage Fracture Stimulation Method Using CT Perforating and Annular Path Pumping SPE 96732 (2005)
The present invention goes in the opposite direction of the application entitled Fracturing with Telescoping Members and Sealing the Annular Space in that it deliberately leaves a gap to the formation such as in open hole so that there is a jetting action of velocity effects on the borehole wall which starts the fractures. Rather than bridging an annular gap from the string to the borehole wall whether with fixed or movable nozzles the present invention directs fluid velocity at the borehole wall to accomplish the fracturing.
Those and other features of the present invention will be more readily understood to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings that are not labeled prior art while understanding that the full scope of the invention is determined by the literal and equivalent scope of the appended claims.